1. Field of the Invention
The present invention relates to a method of growing single crystals. More specifically, the present invention relates to a method of growing substantially stoichiometric lithium niobate (LN) and lithium tantalate (LT) single crystals.
2. Description of the Related Art
LN and LT crystals have been widely used as a photoelectric material, especially for photo-transmission, audio and visual appliance, double-frequency laser, and optical storage medium. A conventional method of growing LN and LT crystals, called as Czochralski method (Cz method), is well known in the art and as described hereafter.
FIG. 1 and FIG. 2 respectively show phase diagrams of a congruent composition with lithium and niobium or lithium and tantalium. Here, a composition with a stoichiometric/non-stoichiometric amount of reactants is also called a stoichiometric/non-stoichiometric composition. That is, in the congruent composition, the mole ratio of lithium to niobium or lithium to tantalium is not about 50%. Therefore, a crystal body obtained by the conventional Cz method usually has a non-stoichiometric composition. However, stoichiometric LN and LT crystals are preferable because of their superior photoelectric properties. A method of continuously growing a stoichiometric LN and LT is thus needed.
Recently, a double-crucible method has been proposed by National Institute for Research in Inorganic Materials (NIRIM), Japan. FIG. 3 is a schematic, cross-sectional view of an apparatus used in a conventional double-crucible method.
In FIG. 3, a melt with a lithium-rich composition for growth of crystals is placed in two separate crucibles. A conventional crystal growing apparatus 300 includes a chamber 302, an external crucible 304, an inner crucible 306, a heater 308, and a crystal pulling system 310. The crystal growing apparatus 300 is provided with a powder feeding system 312 for automatically charging the raw material having the stiochiometric composition. The external crucible 304 is arranged inside the chamber 302. The inner crucible 306 is arranged in the external crucible 304 and has a small opening 314 on the sidewall of the inner crucible 306 near a bottom thereof. The heater 308 surrounds the chamber 302, including a bottom and a sidewall thereof. The powder feeding system 312 is arranged outside the heater 308 and is provided with a tube 318 extending into the chamber 302. An outlet of the tube 318 is located between the external crucible 304 and the inner crucible 306. The crystal pulling system 310 is located above the inner crucible 306.
When the crystal growing process is performed, the solid raw material with about 58% Li is melted and entered into the inner crucible 306 and the external crucible 304 inside the chamber 302. Then, a crystal seed 320 is placed in the crystal pulling system 310 and dipped in the melt 322 within the inner crucible 306. The crystal seed 320 is pulled up at a constant speed while being rotated to grow a crystal body 324. As the crystal body 324 is gradually grown, the powder material 326 is added into the melt 328 within the external crucible 304 at a rate that can compensate the consumed material consumption. Meanwhile, the powder material 326 added in the melt 328 is continuously melted by the heater 308. The melt 328 flows into the inner crucible 306 through the opening 314 of the inner crucible 306. Thereby, the composition of the crystal body 324 can be constant.
The double-crucible method grows stoichiometric lithium niobate (LiNbO3, LN) and lithium tantalate (LiTaO3, LT) single crystals by keeping the composition of the melt 422 in the inner crucible 306 at point A of the phase diagram of FIG. 1 and FIG. 2. Theoretically, the stoichiometric crystals can be obtained as long as the charged amount of powder stuff 326 is precisely controlled to grow the crystal body 324. Automatically feeding powder stuff is requisite for the double-crucible method to grow the stoichiometric crystals. However, it needs high level technology and high production cost. Further, in consideration of thermal aspect, there is a significant difference between the melting points of the melt 322 and 328 respectively in the inner crucible 306 and the external crucible 304, which causes the control of the heater 308 difficult. Therefore, the double-crucible method is difficult to be commercialized at a large scale.